1. Field of Invention
The present invention relates to an objective lens for a microscope of the infinity-correction type, and more particularly to an objective lens of a retrofocus optical system.
2. Description of Related Art
Recently, the demand for an objective lens of very high magnification, having a comparatively large numerical aperture (NA) for the inspection of integrated circuit (IC) wafers has increased, as IC wafers become increasingly integrated. In addition, more work is being performed by microscopic inspection, such as with devices for analyzing defective semiconductors. Where the conduction inspection is performed through a microscope, the objective lens of the microscope must have, not only very high magnification and resolution, but a long working distance.
In view of the above, the Applicant proposed an objective lens, as shown in the Japanese Laid Open Patent Application 62-62317. The objective lens is a plan apochromatic lens and has a magnification of 100 times, numerical aperture of 0.7 and working distance of 8.1 mm without deteriorating the spherical aberration, astigmatism or other characteristics by setting the optical constant of each lens of a predetermined range. However, in order to more efficiently inspect under a microscope, the invention described is not sufficient. It is desirable that the objective lens have a magnification of the order of 200 times and the working distance of more than 10 mm.
Other objective lenses are disclosed in Japanese Laid Open Patent Application 60-209715, U.S. Pat. No. 4,588,264, and U.S. Pat. No. 5,132,845.
The invention of Japanese Laid Open Pat. Application 60-209715 provides an objective lens having a high magnification. However, simply increasing the magnification causes the working distance to decrease.
U.S. Pat. No. 4,588,264 discloses an objective lens having magnification of 60 times, NA of 0.7 and working distance of 5 mm.
U.S. Pat. 5,132,845 discloses an objective lens having magnification of 250 times, NA of 0.9 and working distance of 1.2558 mm.